Run spark test when it quits. Take a spare spark plug, and as soon as it quits, attach the spare spark plug to the spark plug wire, and with it grounded, see if you have spark when cranking. If no spark, your coil is failing and needs to be replaced. If it has spark, then check the fuel solenoid in the base of the carburetor, it may not be opening, cutting off the fuel supply. You should hear it click if you turn key on and off. No click, no fuel. It has to have 12 volts to work!